UNITED NATIONS — Careful not to blame either side for a deadly chemical weapon attack, U.N. inspectors reported Monday that rockets loaded with the nerve agent sarin had been fired from an area where Syria’s military has bases, but said the evidence could have been manipulated in the rebel-controlled stricken neighborhoods.

The U.S., Britain and France jumped on evidence in the report — especially the type of rockets, the composition of the sarin agent, and trajectory of the missiles — to declare that President Bashar Assad’s government was responsible.

Russia, Syria’s closest ally, called the investigators’ findings “deeply disturbing,” but said it was too early to draw conclusions. The Syrian government’s claims that opposition forces were responsible for the attack “cannot be simply shrugged off,” Russian Ambassador Vitaly Churkin insisted.

The conclusions represented the first official confirmation by impartial scientific experts that chemical weapons were used in Syria’s civil war, but the inspectors’ limited mandate barred them from identifying who was responsible for the Aug. 21 attack.

“This is a war crime,” U.N. Secretary-General Ban Ki-moon told the Security Council when he presented the report. “The results are overwhelming and indisputable. The facts speak for themselves.”

Ban called it “the most significant confirmed use of chemical weapons against civilians since Saddam Hussein used them” in Halabja, Iran, in 1988, and “the worst use of weapons of mass destruction in the 21st century.”

The deep division between Western backers of rebels seeking to overthrow Assad and Russian and Chinese supporters of the regime has paralyzed the U.N. Security Council since the Syrian conflict began 2ﾽ years ago.

Even though the United States and Russia agreed Saturday on the framework to put Syria’s chemical weapons stockpile and precursors under international control for future destruction, their top diplomats were at odds Monday over a new Security Council resolution that would make the deal legally binding — and whether there should be a reference to possible military enforcement if Syria doesn’t comply.

After months of negotiations, the U.N. inspectors went to Syria to visit the sites of three alleged chemical attacks earlier this year and were in the capital of Damascus on Aug. 21 when reports and videos began surfacing of a shelling attack in which victims experienced shortness of breath, disorientation, blurred vision, nausea, vomiting, weakness and a loss of consciousness.

They finally gained access to three towns where the Aug. 21 attack occurred, and on one occasion their convoy was hit by sniper fire, but the inspectors were nonetheless able to collect a large amount of material and talk to survivors and witnesses.

“The environmental, chemical and medical samples we have collected provide clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin were used ... in the Ghouta area of Damascus,” their report said.

“The conclusion is that chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic, also against civilians, including children, on a relatively large scale,” they said. “This result leaves us with the deepest concern.”

The rebels and their Western and Arab supporters have blamed Assad’s regime for the attack in the rebel-controlled area of Ghouta. The Syrian government insists the attack was carried out by rebels. The U.N. report mentions the Ghouta areas of Ein Tarma, Moadamiyeh and Zamalka, all of which were featured in videos of victims that emerged after the attack.

The U.N. report did not mention how many people were killed in the Aug. 21 attack.

The U.S. said more than 1,400, but other death toll estimates have been far lower.

The report cited the following evidence to support its conclusions:

• Rockets and fragments were found to contain sarin. “Several surface-to-surface rockets capable of delivering significant chemical payloads were identified and recorded at the investigated sites,” the investigators said.

• Close to the impact sites, in the area where people were affected, inspectors collected 30 soil and environmental samples — far more than any previous U.N. investigation — and in a majority of the samples, “the environment was found to be contaminated by sarin,” its by-products, and “other relevant chemicals, such as stabilizers.”

• Blood, urine and hair samples from 34 patients who had signs of poisoning by a chemical compound provided “definitive evidence of exposure to sarin by almost all of the survivors assessed.”

• More than 50 interviews with survivors and health care workers “provided ample corroboration of the medical and scientific results.”

“The large-scale use of sarin, the direction of the rocket attacks, and kind of rockets used in the attacks all point to use by Assad’s forces beyond reasonable doubt,” said Daryl Kimball, executive director of the Washington-based Arms Control Association.

“The conclusions reached by the United States and European governments would now appear to have received corroboration by a source the Russians and Syrians will have trouble discrediting,” Kimball said.

The inspectors described the rockets used to disperse the sarin as a variant of an M14 artillery rocket, with either an original or an improvised warhead. The report said the rockets that hit two of the suburbs — Zamalka and Ein Tarma — were fired from the northwest, but it didn’t say who launched them.

The inspectors did not provide a location for the rockets’ launch site, but Qassioun Mountain, where the Syrian military is known to have bases, is roughly northwest of both suburbs.

“To put it in perspective, just on those rocket samples that they were able to examine, they had a payload of a total of 350 liters, which is 35 times the amount that was used in the Tokyo subway” in 1995, he said, adding that the inspectors also confirmed “that the quality of the sarin was superior” both to that used in Tokyo and also to what was used by Iraq against Iran.

U.S. Ambassador Samantha Power noted that chief inspector Ake Sellstrom said the weapons “were professionally made.”

“It defies logic that the opposition would have infiltrated the regime-controlled area to fire on opposition-controlled areas,” she said. “Only the regime could have carried out this large-scale attack.”

But Churkin wondered why there were no reports of casualties among opposition fighters if government forces fired rockets filled with sarin to try to oust opposition groups from the area.

“Is it theoretically possible to fire five or six rockets and miss your opponent?” he asked.

The inspectors cautioned that the five sites they investigated had been “well- traveled by other individuals prior to the arrival of the mission.”

“During the time spent at these locations, individuals arrived carrying other suspected munitions indicating that such potential evidence is being moved and possibly manipulated,” the report said. The areas were under rebel control, but the report did not elaborate on who the individuals were.

Associated Press writers John Heilprin in Geneva, Matthew Lee in Paris, Peter Spielmann at the United Nations, and Albert Aji in Damascus, Syria, contributed to this report.